Dosing mechanisms are commonly used for the delivery of metered doses of therapeutic material from a cartridge (or other reservoir). For example a dosing mechanism may be provided as part of a pen injector in which a cartridge containing therapeutic material, for example insulin, is received in (or formed by) a pen body.
Such dosing mechanisms are arranged to administer a plurality of repeatable single metered doses. The volume of each single dose may be variable and, therefore, a dose selector, for example a dial, may be provided for adjusting the action of the dosing mechanism.
A dose delivery mechanism is generally provided which expels a selected dose in response to a used pressing a release button. The dose delivery mechanism is moveable between a primed position in which it is ready to be released and an end position in which a dose has been expelled. A return spring (or other biasing means) may be provided which urge the dose delivery mechanism to its primed position. Alternatively, the dose delivery mechanism may be biased towards its end position and manual means may be provided for priming the mechanism (for example the release mechanism may be primed such that a catch holds it in position against a biasing spring and the release button may release the catch).
The applicants have now found that in some dosing mechanisms (and particularly in mechanisms that are biased towards a primed position) there is a risk that a user may pause during delivery of a medicament and allow the dosing mechanism to return (or partially return) towards its primed position. This results in a risk that an additional dose (or partial dose) may be delivered over and above the desired single metered dose.
Accordingly, embodiments of the invention seek to provide an improved dosing mechanism in which the risk of a partial dose delivery is reduced or removed.